When Hurricanes Go Extratropical

By
Steve Tracton

In a recent post, I recounted the unusual redevelopment of the remnants of Tropical Storm Erin over Oklahoma in August 2007, a few days after it had made landfall along the Texas coast. This inland "ghost" of Erin did not fit the criteria of any traditional storm type -- tropical, extratropical or subtropical -- according to the National Hurricane Center.

While not the case with Erin, at least not officially, about 25 percent of all hurricanes and typhoons moving northward of 30-40 degrees north latitude do become extratropical storms, in some cases larger and stronger than the original storm, through a redevelopment process known as extratropical transition.

Extratropical is a fancy name for the kind of storm that we are all used to -- the typical storm that moves across the United States with a cold front and warm front extending outward from a low-pressure center.

Keep reading for more on storms that go extratropical. Also, see our full forecast through the weekend and beyond.

Relative to tropical systems, extratropical storms are different in structure, size, and in the physical processes that govern their development and motion. For example, tropical storms and hurricanes have centers that are warmer than the surrounding air, no fronts, and winds that are strongest near the ground. Extratropical storms, on the other hand, have centers that are colder than the surrounding air, fronts, and winds that are strongest high up in the atmosphere.

The track of Hurricane Hazel in 1954. Courtesy NOAA.

The classic example of a storm going from tropical to extratropical is Hurricane Hazel, which in 1954 transformed into an extratropical cyclone that behaved much like a Nor'easter after crossing the coast near the North Carolina/South Carolina border. The extratropical version of Hazel produced sustained winds of 78 mph in D.C., and wind gusts to 112 mph in New York City -- over 200 miles from the storm's center. That remains the highest wind speed on record for the Big Apple.

Indeed, hurricane-force winds were maintained even after Hazel's remnants had moved 600 miles inland. Had Hazel tracked a bit further east and approached New York just west of the city, the unique configuration of the shoreline might have resulted in a surge of waters into New York harbor that could have inundated much of the city, a NASA/Columbia University study showed.

Extratropical storms that were once tropical in nature often produce heavy rainfall and widespread floods over a much broader expanse than the parent storm (minus the coastal flooding from storm surge). With Hazel, heavy rains of up to 11 inches occurred as far north as Toronto, Canada, where 81 people were killed and entire neighborhoods were washed away.

Hurricanes in the west Pacific (actually, they're called typhoons there) transform into extratropical storms more frequently than do Atlantic-born hurricanes. I can hear you thinking, "So what, that doesn't affect me." Wrong! Oh, yes it does! While the impacts of Hazel were felt directly and immediately over much of the eastern U.S., seemingly far-removed events occurring in the west Pacific can often affect the weather locally within a week.

This is not a "believe-it-or-not" wonder. It's a natural phenomenon that is not well understood or predicted. Curious? Stay tuned for more in a future post.

My mother used to tell me stories about her Hurricane Hazel experiences. She lived near the (now) intersection of I95 and I40 in NC and was let out from school in the eye of the storm and was not home long when the other side hit.

Tropical warm-core systems derive their energy primarily through the heat-engine mechanism by which the latent heat of condensation is converted to kinetic (wind) energy. Heat of condensation creates the pressure differential which drives the winds.

Extratropical storms derive their energy primarily from the temperature difference between two air masses of contrasting temperatures on opposite sides of a frontal boundary. Specifically the temperature differences create or intensify an upper-air westerly jet which then causes the pressure differential which drives the extratropical winds. [In addition, condensation in the warmer, generally more humid air mass on the warm side of the frontal boundary may add its energy from heat of condensation to the extratropical mix. This is particularly evident in our Atlantic coast nor'easters, where land/ocean temperature differences add further to the mix.]

Why are tropical cyclones often more intense than extratropical storms? The reason is that generally extratropical storms cover a far greater surface area. Although there may be more energy available in an extratropical storm, that energy is dispersed over a far wider area.

"Hazel" represents a system when, for a period of time, both tropical and extratropical processes were functioning during the transition. The energy from both processes overcame the general loss of energy due to friction associated with overland passage; thus the storm maintained hurricane intensity inland.

One final note: Although tropical systems generally are frontless, they tend to develop along the Intertropical Convergene Zone or "meteorological equator" which often behaves like a front. The major difference between the ITCZ and an extratropical front lies in the fact that temperatures and humidities on both sides of the ITCZ are generally the same or nearly so. The ITCZ is the abode of the "equatorial" air mass in which moisture can rise to levels higher than that even associated with tropical air.

Anyone know more details on the conditions in place at the time of Hazel that allowed that inland track and strong extratropical development to happen? It would be interesting to see what factors played into it and how a similar storm could happen again. (Once a century event, maybe).

The extratropical transition (ET) of Hurricane Hazel might be considered another "perfect storm" in that the confluence of ingredients was critically dependent on the timing, location and specific characteristics of two independent major weather systems: hurricane Hazel crossing the coastline and the approach from the west of an intensifying upper-level disturbance (trough). Only a few hours difference in tiiming or a ~ 50km difference in the convergence of the separate physical mechanisms (see El Bombo's comments above) driving each would be the difference between either no (or weak) ET and the "explosive development that actually occurred. There is little question that the upper-level trough would have generated a fairly intense extratropical cyclone (e.g., nor'easter) independent of Hazel. But, merger of this development with substantial moisture content and pre-existing circulation of Hazel was equivalent to throwing gasoline on a fire.

While ET is not all that uncommon, the intensity of the Hazel ET is relatively rare, but certainly not unprecedented - and this is especially so in the ET of typhoons in the Pacific. The nature,characteristics, and predictability of ET remains a topic of intense research. Indeed, this fall and winter there will be a major field program to investigate ET in the Pacific. As I said in the post, these systems can and do often have pronounced implications on the weather right here.

There's a lot more to this subject. I'll be glad to discuss or chat offline (s.tracton@hotmail.com) for anyone who might wish to do so. The degree of "geekiness" is up to you.